1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine with a three-way catalyst for purification of exhaust gas installed on an exhaust system, and more particularly, it relates to a new technique to detect the degradation of the three-way catalyst in a reliable manner.
2. Description of the Related Art
In general, in internal combustion engines, a three-way catalyst is used to purify harmful components of an exhaust gas. The three-way catalyst has an oxygen occlusion capability to keep the atmosphere inside the three-way catalyst at a stoichiometric air fuel ratio by occluding oxygen in the exhaust gas when the air fuel ratio of the exhaust gas is leaner than the stoichiometric air fuel ratio, while releasing the oxygen occluded therein when the air fuel ratio of the exhaust gas is richer than the stoichiometric air fuel ratio.
In addition, the three-way catalyst also has a capability to oxidize HC and CO among three harmful components contained in the exhaust gas and to reduce NOx, thereby purifying these respective components into harmless gases. Further, since the purification ability of the three-way catalyst becomes maximum in the vicinity of the stoichiometric air fuel ratio, the exhaust gas is excellently purified by combining the oxygen occlusion ability and the purification ability of the three-way catalyst with each other.
However, when the exhaust gas becomes leaner than the stoichiometric air fuel ratio to cause the amount of oxygen occluded in the three-way catalyst to exceed the oxygen occlusion capacity thereof, the atmosphere in the three-way catalyst becomes no longer kept at the stoichiometric air fuel ratio, so the NOx purification rate of the catalyst is deteriorated to a remarkable extent.
In addition, when the exhaust gas becomes richer than the stoichiometric air fuel ratio so the amount of oxygen occluded in the three-way catalyst becomes lacking or insufficient, the atmosphere in the three-way catalyst can not be kept at the stoichiometric air fuel ratio, thus deteriorating the purification rate of HC and CO. Here, it is known that as the three-way catalyst is deteriorated, the oxygen occlusion capacity thereof decreases, thus worsening the purification performance thereof.
Accordingly, there has been proposed a control apparatus for an internal combustion engine in which a pair of air fuel ratio sensors are arranged at an upstream side and at a downstream side, respectively, of a three-way catalyst so as to directly measure an oxygen occlusion capacity thereof to detect the degradation of the three-way catalyst (see, for instance, a first patent document: Japanese patent No. 2812023). In this case, in changes of the air fuel ratios at the upstream and downstream sides of the three-way catalyst and in a change of the concentration of harmful components in the exhaust gas at the downstream side of the three-way catalyst, the air fuel ratio at the upstream side of the three-way catalyst is switched from a predetermined air fuel ratio, which is preset to a lean side, into a first prescribed air fuel ratio, which is preset to a rich side, across the stoichiometric air fuel ratio.
At this time, even if the air fuel ratio at the upstream side of the three-way catalyst changes into the rich side, the oxygen adsorbed and held in the three-way catalyst is released. As a result, the air fuel ratio at the downstream side of the three-way catalyst is first maintained at the stoichiometric air fuel ratio only for a first predetermined period of time, and thereafter reaches a first air fuel ratio which is at the rich side. Subsequently, the air fuel ratio at the upstream side of the three-way catalyst is switched from the predetermined air fuel ratio, which is preset at the rich side, into a second prescribed air fuel ratio, which is preset at the lean side, across the stoichiometric air fuel ratio.
At this time, oxygen is adsorbed and held in the three-way catalyst, contrary to the above case. As a result, the air fuel ratio at the downstream side of the three-way catalyst is first maintained at the stoichiometric air fuel ratio only for a second predetermined period of time, and thereafter reaches the second air fuel ratio at the lean side. Hereinafter, an absolute amount of the oxygen adsorbed and held by the three-way catalyst is calculated from a difference between the switched air fuel ratio and the stoichiometric air fuel ratio, and from the amount of the exhaust gas that has passed through the three-way catalyst for the first or second predetermined period of time, so that the degradation level of the three-way catalyst is detected from the absolute amount of adsorbed oxygen thus calculated (see FIG. 6 of the above-mentioned first patent document).
In the known control apparatus for an internal combustion engine as referred to above, after the amount of oxygen occluded in the three-way catalyst has been decreased to zero (or saturated) without fail, the air fuel ratio at the upstream side of the three-way catalyst is switched, and then the absolute amount of the oxygen adsorbed and held by the three-way catalyst is calculated. Accordingly, the purification rate of the three-way catalyst in a period of time in which the amount of the oxygen occluded in the three-way catalyst is being decreased to zero or saturated is reduced, thus posing a problem that the exhaust gas is deteriorated.